Open Users & Groups preferences for me

Select the user, then click Login Items.

Do one of the following:

Click the Add button below the list on the right, select a document, or folder, then click Add.

Select the name of the item you want to prevent from opening automatically, then click the Remove button below the list on the right.

If you don’t want an item’s windows to be visible after login, select Hide.

Note: Hide does not apply to servers, which always appear in the Finder after login.

(from Mac Help...)
A note from me: You might have to unlock the pane to make changes. And you might want to re-lock it, to keep your change(s).

Cheers!